To elaborate: A single IC die (a "chiplet" when stacked on others) is made of a layer of transistors and wiring up to several hundred nanometers thick on top o

To elaborate: A single IC die (a "chiplet" when stacked on others) is made of a layer of transistors and wiring up to several hundred nanometers thick on top of a much thicker (several hundred micrometer) silicon wafer substrate. Yep, stacking these dies ("chiplets") on top of one another is one form of 3D, and is definitely useful: IIRC, AMD's recent popular Ryzen 3000 line uses chiplets, and the Raspberry Pi's RAM chip is stacked on top the CPU (in a much cruder post-IC-manufacturing assembly process). The shorter distance between dies (compared to placing them in separate plastic packages on a PCB) can lead to maybe an order of magnitude improvement (in speed/power/etc). It's hard to stack more than a few layers. The most ambitious form of 3D design (and the one most people probably think of) is multiple thin layers of transistors on a single die ("monolithic 3D"); this would give maybe another order of magnitude improvement. Monolithic 3D memory chips are becoming popular (V-NAND, etc), with the most recent at ~100 layers. Monolithic 3D CPUs are still an unsolved problem because they need different, more difficult process steps and better heatsinks (but we're close!).